It is desirable for tires to have good wet skid resistance, low rolling resistance, tear strength, and good wear characteristics. It has traditionally been difficult to improve the wear characteristics of a tire without sacrificing wet skid resistance and traction characteristics. These properties depend, to a great extent, on the dynamic viscoelastic properties of the rubbers utilized in making the tire. Such viscoelastic properties are controlled largely by the microstructure, macrostructure, and the degree of branching of the elastomer.
Rubbers having a high rebound have traditionally been utilized in making tire tread rubber compounds to reduce the rolling resistance and to improve the treadwear characteristics of tires. On the other hand, rubbers that undergo a large energy loss have generally been utilized in the tread of the tire to increase the wet skid resistance of a tire. To balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber (SBR) and polybutadiene rubber are commonly used as a rubbery material for automobile tire treads.
It has been previously generally understood to be desirable for styrene-butadiene rubber to have a high level of vinyl content (1,2-microstructure) when utilized in tire tread compounds. However, in contrast to providing high vinyl content rubbery polymers, it is believed that rubbery polymers having low, or lower, vinyl microstructure content may provide a more desirable balance of tire tread properties. Such rubbery polymers can be synthesized by solution polymerization in the presence of a catalyst system.